This allows structs to use the default value for each field defined in
the struct’s `std::default::Default` implementation, rather then the
default value for the field’s type.
```
struct StructDefault {
a: i32,
b: String,
}
impl Default for StructDefault {
fn default() -> StructDefault {
StructDefault{
a: 100,
b: "default".to_string(),
}
}
}
```
The code above will now return `100` for field `a` and `”default”` for
`b`, rather then `0` and `””` respectively.
